The present invention relates to a code division multiple access (CDMA) mobile communication system and, more particularly, to a radio channel multiplex communication method ,a mobile unit, and a base station which are for use in the CDMA mobile communication system.
A mobile communication system is well known which comprises a base station control device connected to a mobile communication switching unit via a wire channel, a plurality of base stations connected to the base station control device via wire channels, and a plurality of mobile units located in a service area of each base station connected to the base station via a radio channel.
Moreover, as a method of multiplexing the radio channel between each base station and the plurality of mobile units to obtain multiple channels, together with frequency division multiple access (FDMA) and time division multiple access (TDMA), code division multiple access (CDMA) is known in which information bits to be communicated are subjected to spectrum diffusion and multiplexed in the same carrier wave by a diffusion code pattern formed of several tens to several thousands of bits.
The CDMA mobile communication system has advantages in that it is strong against a narrow bandwidth disturbance and that a multiplicity of radio channels can be accommodated in the same carrier wave.
In the CDMA mobile communication system, the number of radio channels is determined in the service area by the number of diffusion codes in the CDMA mobile communication system. The number of diffusion codes, which can be used, is limited and limits the number of radio channels which can simultaneously be secured. Therefore, when the traffic amount is large, there are a multiplicity of mobile units which cannot simultaneously communicate.
Particularly in the CDMA mobile communication system, since a method of connecting a plurality of radio channels to one mobile unit is employed in order to prevent communication quality from being deteriorated, there are a multiplicity of mobile units which cannot simultaneously communicate. To prevent this problem, for example, a method of multiplexing frequency to increase the resources of radio channels is proposed, but this method has a problem that the equipment is enlarged, and other problems.
Additionally, a technique of further time-division multiplexing a CDMA signal is disclosed, for example, in Japanese Patent Application Laid-Open No. 252490/1997 titled xe2x80x9cRadio Base Stationxe2x80x9d (hereinafter referred to as the first prior art), Japanese Patent Application Laid-Open No. 327073/1997 titled xe2x80x9cPilot Channel Arrangement and Transmission Method in CDMA Mobile Communication Systemxe2x80x9d (hereinafter referred to as the second prior art), Japanese Patent Application Laid-Open No. 318927/1994 titled xe2x80x9cCommunication Method and Systemxe2x80x9d (hereinafter referred to as the third prior art), Japanese PatentApplication Laid-Open No. 237214/1994 titled xe2x80x9cMobile Communication Systemxe2x80x9d, and the like.
For example, in the first prior art, for the purpose of the time division multiple access of a channel to enhance the use efficiency of the channel, a radio base station comprises spectrum diffusion modulating means for performing spectrum diffusion/modulation based on a diffusion code selected by control means, time division multiplexing means for time-division multiplexing the signal modulated by the spectrum diffusion modulating means to dispose the signal in a time slot, and transmitting means for transmitting the time-division multiplexed and disposed signal.
Moreover, in the second prior art, an object is to provide a pilot channel arrangement and transmission method in CDMA mobile communication system, in which a descending radio channel is time-division multiplexed, the pilot channel is efficiently assigned, and the interference power of the pilot channel to other cells can be reduced. There is provided a pilot channel arrangement and transmission method in which a plurality of cells are connected to base stations, the base stations are modulated with the same frequency, the pilot channels diffused with different assigned diffusion codes are transmitted, and a mobile station judges a cell located in an area by receiving the pilot channel. The method comprises the steps of time dividing a descending radio channel leading toward the mobile station from the base station into a plurality of time slots to perform the time division multiplexing, and assigning one of the plurality of time slots as the pilot channel.
Furthermore, in the third prior art, an object is to provide a communication method and system in which the number of users in one network or cell can be increased. Data is transmitted using CDMA/TDMA or TDMA/CDMA combination for multiplex use, and a complementary code set (completely orthogonal codes or mutually non-acting codes) is used as a spreading code.
To apply this method to the existing or planned TDMA system, CDMA section is added to the system. Moreover, to increase the number of users, diffusion coding CDMA multi-access can be utilized in all time slots of TDMA frame in the above-described system.
Therefore, any power control method or multi-access interference cancel method becomes unnecessary.
Moreover, in the fourth prior art, an object is to provide a mobile communication system which has a large capacity, which is strong against narrow bandwidth disturbance, and which can change the data rate. When the spectrum diffusion system is used in the radio communication system, and a plurality of data strings are sent on the same carrier wave by changing a diffusion code pattern by a diffusion circuit, the data string of the same diffusion code pattern is time-division multiplexed in a burst generating circuit and a parallel/serial converting circuit, and the communication channels are assigned to the slots. In this case, by providing the data string with a spare time slot to avoid the disturbance, the advantage that the system is strong against the narrow bandwidth disturbance of the spectrum diffusion system, and the advantage that the data rate of time division multiplex system can be varied can both be utilized. More communication channels can be secured with less diffusion code patterns as compared with the prior art, and the deterioration of a desired wave reception level by other diffusion codes in the same carrier wave can be suppressed.
As described above, there have been proposed various techniques of increasing the radio channels with the signals obtained by further time-division multiplexing the CDMA signals. When the technique is used in the mobile communication system, particularly the synchronization of the time slots on the transmission and reception sides is a problem.
When the time division multiplex signal is used, needless to say, it is important to accurately obtain the synchronization of the time slots on the transmission and reception sides. However, the conventional mobile unit or base station is not constituted to obtain accurate time information by itself. For example, the time information obtained by the base station control device is received with a control signal as occasion demands. Therefore, it is difficult to obtain accurate synchronization of the time slots on the transmission and reception sides. As described later, a guard timing for absorbing synchronization deviation has to be set to be large, which deteriorates the communication efficiency.
It is therefore an object of the present invention to provide a CDMA mobile communication system, in which more radio channels can be secured without increasing the frequency for use.
It is another object of the present invention to provide a CDMA mobile communication system of the type described, in which synchronization deviation is suppressed.
It is still another object of the present invention to provide a CDMA mobile communication system of the type described, in which guard timing can be reduced.
It is yet another object of the present invention to provide a mobile unit which is suitable for use in the CDMA mobile communication system.
It is a further object of the present invention to provide a base station which is suitable for use in the CDMA mobile communication system.
Other objects of the present invention will become clear as the description proceeds.
According to the present invention, there is provided a radio channel multiplex communication method for a code division multiple access mobile communication system including a bas station and a plurality of mobile units and using a communication signal, modulated and multiplexed with different diffusion codes, in data communication between the base station and each of the mobile units. The radio channel multiplex communication method comprises the steps of producing, as the communication signal, a time division multiplex signal with a repeated frame having a plurality of time slots, assigning the time slots to different radio channels, respectively, obtaining a reference clock signal of a global positioning system, producing a slot synchronous signal in accordance with the reference clock signal, and synchronizing the time slots between the base station and each of the mobile units in accordance with the slot synchronous signal.
It may be arranged that the radio channel multiplex communication method further comprises the steps of obtaining measured position information of the global positioning system processing the measured position information to produce communication distance information representative of a distance between the base station and each of the mobile units, delaying the slot synchronous signal in accordance with the communication distance information to produce a delayed slot synchronous signal, and using the delayed slot synchronous signal to synchronize the time slots used in a reception side of the data communication.
It may be arranged that the repeated frame has an ascending frame and a descending frame, the radio channel multiplex communication method further comprising the step of deviating the ascending and the descending frames to each other by one of the time slots.
It may be arranged that the radio channel multiplex communication method further comprises the step of making the time slots be different from one another in length.
According to the present invention, there is provided a radio channel multiplex communication method for a code division multiple access mobile communication system including a base station and a plurality of mobile units and using a communication signal, modulated and multiplexed with different diffusion codes, in data communication between the base station and each of the mobile units. The radio channel multiplex communication method comprises the steps of producing, as the communication signal, a time division multiplex signal with a repeated frame having a plurality of time slots, assigning the time slots to different radio channels, respectively, obtaining a reference clock signal of a global positioning system, producing a slot synchronous signal in accordance with the reference clock signal, and synchronizing the time slots between the base station and each of the mobile units in accordance with the slot synchronous signal.
It may be arranged that the GPS receiver further receives measured position information of the global positioning system, the base station further comprising information processing means connected to the GPS receiver for processing the measured position information to produce communication distance information representative of a distance between each of the mobile units and the base station and delaying means connected to the information processing means and the synchronizing means for delaying the slot synchronous signal in accordance with the communication distance information to produce a delayed slot synchronous signal, the synchronizing means synchronizing the time slots in accordance with the delayed slot synchronous signal at a reception side of the base station.
According to the present invention, there is provided a base station for use in a code division multiple access mobile communication system including a plurality of mobile units and using a communication signal, modulated and multiplexed with different diffusion codes, in data communication between the base station and each of the mobile units, the communication signal being a time division multiplex signal with a repeated frame having a plurality of time slots which are assigned to different radio channels, respectively. The base station comprises a GPS receiver for receiving a reference clock signal of a global positioning system, clock processing means connected to the GPS receiver for processing the reference clock signal into a slot synchronous signal, and synchronizing means connected to the clock processing means for synchronizing the time slots between each of the mobile units and the base station in accordance with the slot synchronous signal.
It may be arranged that the GPS receiver further receives measured position information of the global positioning system, the base station further comprising information processing means connected to the GPS receiver for processing the measured position information to produce communication distance information representative of a distance between each of the mobile units and the base station and delaying means connected to the information processing means and the synchronizing means for delaying the slot synchronous signal in accordance with the communication distance information to produce a delayed slot synchronous signal, the synchronizing means synchronizing the time slots in accordance with the delayed slot synchronous signal at a reception side of the bas station.